fix: As of django-filer 3.0 mptt is not needed as a dependency

[fsbraun]

Description

Describe:

• This PR only automatically adds mptt to INSTALLED_APPS for django-filer, if the installed version of django-filer is below 3.0.
• django-filer as of version 3.0 does not depend on mptt any more.
• Without this change, a ModuleNotFound exception will be raised in an environment without mptt

Checklist

• I have read the contribution guide
• Code lint checked via inv lint
• changes file included (see docs)
• Usage documentation added in case of new features
• Tests added

[codecov]

Codecov Report

Patch coverage: 33.33% and project coverage change: -0.24 ⚠️

Comparison is base (1bba8eb) 92.68% compared to head (264b1bd) 92.45%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #224      +/-   ##
===========================================
- Coverage    92.68%   92.45%   -0.24%     
===========================================
  Files            9        9              
  Lines          793      795       +2     
  Branches       124      125       +1     
===========================================
  Hits           735      735              
- Misses          22       23       +1     
- Partials        36       37       +1     

Flag	Coverage Δ
unittests	92.45% <33.33%> (-0.24%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
app_helper/utils.py	92.97% <33.33%> (-0.78%)	⬇️

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[yakky]

@fsbraun thanks, I merged it with #226 and released version 3.3.1